1 Technical Field of the Invention
The present invention relates generally to an engine control apparatus for multi-cylinder compression ignition internal combustion engine which is designed to correct injection timings based on the time of ignition of fuel in the engine.
2 Background Art
Typical multi-cylinder compression ignition internal combustion engines such as diesel engines have installed therein fuel injectors to inject fuel directly into cylinders thereof to produce combustible air-fuel mixture. Engine control systems for such a type of engines are usually designed to correct times of injection of the fuel into the cylinders (i.e. injection timings) based on the status of combustion of the engine in order to ensure desired levels of output of and quality of emissions from the engine. Specifically, the engine control systems work to sample an output from a combustion pressure sensor installed in the engine to determine an actual time of ignition of the fuel in the engine and correct the injection timings so as to bring the actual time of ignition into agreement with a target value.
Use of combustion pressure sensors in all the cylinders of the engine to correct the injection timings independently on a cylinder basis result in disadvantage in terms of cost. In order to alleviate this problem, there may be proposed systems which have a combustion pressure sensor installed only in one of cylinders of the engine to sample an actual time of ignition of fuel therein, determine the injection timing for the one of the cylinders so as to compensate for an ignition lag that is a difference between the actual time of ignition, as sampled, and a target time, and correct injection timings for the other cylinders based on the ignition lag under assumption that the other cylinders usually undergo the same ignition lag.
When the time of ignition of the fuel is retarded from some time point, it usually results in an undesirable drop in output from the engine. This is because the ignition of the fuel upon a drop in pressure in the cylinder results in a lack in utilizing combustion energy or a misfire in the course of combustion of the air-fuel mixture without adequate expansion thereof, leading to a reduction in the combustion energy.
In order to avoid the above problem, Japanese Patent First Publication No. 2000-130224 teaches an engine control system for internal combustion engines equipped with an EGR (Exhaust Gas Recirculation) system working to return or recirculate a portion of exhaust gas of the engine (i.e., EGR gas) into an intake pipe. The engine control system has a combustion pressure sensor installed in only one of cylinders of the engine which is the greatest in ratio of the amount of the EGR gas to the amount of intake air in the engine and works to determine and correct all target times of injection of fuel (i.e., injection timings) into the cylinders based on an actual time of ignition of the fuel, as determined using an output of the combustion pressure sensor. The engine control system is designed based on the fact that the one of the cylinders that is the greatest in the EGR gas-to-intake air ratio is apt to experience a misfire due to a drop in concentration of oxygen in the intake air arising form addition of the EGR gas thereto to correct the injection timings based on the actual time of the ignition in the one of the cylinders in which the combustion pressure sensor is installed to avoid the misfire.
However, fuel injectors have usually individual variability in injecting fuel into cylinders of the engine. A crank angle sensor has also individual variability in measuring the angular position of a crankshaft of the engine arising from manufacturing tolerances thereof. This will result in a variation in time of ignition of the fuel among the cylinders of the engine, which may lead to an undesirable shift of the times of ignition of the fuel in the cylinders in the regarded direction from a time point at which the quality of combustion of the fuel is deteriorated, resulting in a drop in torque, as produced by the engine.